In membrane process industries, membrane cleaning is one of the most important concerns from both economic and scientific points of view. Though cleaning is important to restore membrane performance, the inappropriate selection of cleaning agents may result in unsatisfactory cleaning or irreparable membrane damage. In this study, the cleaning performance was studied by measuring membrane pore size; by positron annihilation spectroscopy and salt rejection as well as by flux measurement. Thin film composite nanofiltration (NF) membranes, supplied by GE Osmonics NF-DK were used in this study. Tests were carried out on virgin membranes. Several different cleaning agents were investigated. Some of them were of analytical grade such as HCl and NaOH. Others such as SDS and mixed agents were of commercial grade already in use in commercial plants. Pore size and salt rejection as well as the flux of virgin membranes before and after chemical cleaning were measured and compared. After the chemical cleaning of the virgin membranes, the membrane pore size was measured revealing interesting results, which may be used to characterize membrane surface cleanliness. The membrane pore size results showed that the cleaning agents affect membrane surface properties by enlarging the pore size of the treated virgin membranes. The flux and rejection were correlated with pore size after low and high pH cleaning, and these findings were investigated and reported in this paper. (C) 2013 Elsevier B.V. All rights reserved.